Foamed beads of various polymers, such as polystyrene, polyethylene and polypropylene, and their use to prepare foamed articles, are known.
It is known that foamed beads and articles obtained from polypropylene have far higher characteristics of chemical inertness, heat-resistance, resilience and impact-resistance than foamed polystyrene.
In the case of polypropylene, however, the process for preparing the beads is either complicated or requires the use of a polymer which is modified by linking side chains in order to improve its melt strength.
In the case of polyester resins, particularly polyethylene terephthalate (PET), the possibility to obtain foamed beads requires the use of resins which have appropriate rheological characteristics in order to be able to foam the particles.
The resins furthermore have relatively high melting temperatures and an equally high crystallinity under the temperature conditions in which it is necessary to work in order to allow the sintering of the beads.
This makes it troublesome to use pre-foamed beads of polyester resin in preparing molded foamed articles.
The production of foamed materials having a complex geometric shape and/or a considerable thickness directly by extrusion-foaming of the polyester resin is difficult to perform.
The need is therefore felt to have foamed beads of polyester resin which can be sinterized easily and effectively in order to be able to prepare said foamed articles.